Such amplifying fibers are conventionally doped with erbium. Initially they were made using lengths that were very short compared with the total length of the connection. They were then "lumped" in amplifiers.
The limitations that need to be taken into account when designing a very long distance connection using erbium-doped amplifying fibers (EDFA) are essentially due to two phenomena:
The accumulation of noise due to amplified simultaneous emission (ASE). In this context, it may be observed that to obtain specified performance (in terms of signal to noise ratio) at the end of a given distance, it is necessary for the output power from the amplifiers (and consequently the mean in-line power) to be greater than a limit value. As the spacing between amplifiers is increased, so said limit value also increases.
Dispersive and non-linear effects. It has been shown that loss of quality in a connection due to these effects increases with increasing mean in-line power.
Various connection configurations have been proposed: in one proposal, amplification is lumped. The connection makes use of lumped amplifiers each integrating all of the active and passive components required for amplification (short amplifying fiber strongly doped with erbium, pump injectors, i.e. pump diodes providing a pump wave, and a multiplexer for injecting said pump wave into the amplifying fiber, . . . ).
A connection of that configuration is described, in particular, in the document "Propagation of signal noise in concatenated erbium-doped fiber optical amplifiers" by C. R. Giles and Emmanuel Desurvie, and published in the Journal of Lightwave Technology, Vol. 9, No. 2, February, 1991.
Another configuration that has been proposed is distributed amplification. This is obtained by compensating the losses to which the signal is subjected by means of a line fiber that is weakly doped and by placing pump injectors along the line, the pump injectors comprising pump diodes and multiplexers.
That connection configuration is described, in particular, in the above-mentioned document.
In order to avoid having mean in-line powers, and thus dispersive and non-linear effects that are too penalizing when using lumped amplification, it is necessary for distances between amplifiers to be short: about 30 km to 50 km.
The distributed amplification configuration is more advantageous from the point of view of noise accumulation. Under such circumstances, the distance between pump injectors is limited by the pump power that needs to be injected into the fiber. 100 km appears to be the maximum distance that can be considered, and requires relatively high pump power to be injected (more than 30 mW).
Another possible configuration is mentioned in patent document EP-A 421 675 (ATT). That configuration comprises amplification that is made more nearly continuous by having varying concentrations of dopant such as erbium between two pump injectors. The concentrations are chosen to make signal power more uniform so as to minimize non-linear effects. Starting from a pump injector, the concentration in successive lengths increases so as to compensate for the decreasing power of the pump light. It is specified that some lengths may have no doping at all.
That configuration suffers from drawbacks comparable to those of the other, previously-proposed configurations.
Particular objects of the present invention are to provide a connection in which the spacing between pump injectors is greater than it is with lumped amplification and in which the pump power required is compatible with system constraints without non-linear effects becoming too troublesome.